Four endearing but rather naughty siblings–Robert, Nan, Timothy, and Betsy– are left to stay with their grandmother while their father is in Egypt with his regiment. “Grandmother said they were insubordinate; Father only thought them high-spirited.” Since the children’s first acts in the book are to run away from grandmother’s house and to “borrow” a pony and cart full of someone’s else’s groceries, I tend to agree with Grandmother. But the children turn out to be charming, nevertheless.
And they don’t stay with Grandmother very long. It’s not much of a spoiler, since the change happens in the second chapter of the book, to tell that the four incorrigible children end up living with their Uncle Ambrose, a Church of England clergyman, former educator, and inveterate bachelor. Uncle Ambrose also claims to dislike children, but he takes his nephews and nieces into his home anyway. And so the adventure begins.
This description is from the review that piqued my interest in the first place, over at Semicolon blog. Definitely go read that review for a top-notch overview.
My brief take is that in many ways it makes me think of E. Nesbit's tales such as The Magic City, The House of Arden, and The Treasure Seekers. Like those, this book takes recognizable fantasy beats and weaves an entirely new and enchanting pattern.
Don't go by the cover which is appalling. Do read it.